The present invention is directed generally to a cartridge and method for breaking and specifically to a cartridge and method for small charge breaking of rock, concrete and other materials.
Small charge breaking has become an important mining technique in recent years compared to conventional breaking due to the reduced toxic gaseous emissions in the mine (which enables the excavation face to be cleared of such emissions rapidly), increased personnel safety due to the use of smaller amounts of energetic materials (and less flyrock), and the ability to have personnel remain safely in the excavation in the general vicinity of the excavation face during breakage.
In a typical small charge breaking process, a hole is formed in the material to be excavated. One or more small charge cartridges are placed into the bottom or toe of the hole, and the hole is stemmed with a suitable material. The stemming material can be a stemming member such as a massive bar and/or a particulate stemming material such as gravel. When the energetic material in the cartridge is initiated, there is a rapid generation of gas and thus a rapid build up of gas pressure near the toe of the hole. Provided that the gas generated is contained for a short period of time, the resulting gas pressure may cause fractures to be propagated from the hole through the material.
A number of problems have been encountered during small charge breaking. By way of example, a relatively high occurrence of misfires has been experienced. The misfires are believed to be due to a number of factors, including water egress into the cartridges and subsequent damage of cartridge components and igniter damage and/or lead breakage during insertion of the cartridge into the hole. Small charge breaking has been relatively inefficient due to an unacceptably low rate of pressure rise in the hole and/or unacceptably high rate of gas loss from the hole due to inefficient sealing of the hole. Small charge breaking cartridges have been relatively expensive to ship due to stringent regulations governing explosive materials. Such costs have significantly increased small charge breaking costs. To mitigate such costs, some cartridge manufacturers have separately shipped the cartridge in parts which are then assembled at the excavation site. Such on-site assembly has had mixed success due to the relative complexity of the cartridge design and the consequent quality control problems. Further problems have been encountered with small charge breaking cartridges xe2x80x9cbindingxe2x80x9d or getting stuck in the holes during cartridge insertion. Finally and relatedly, it has been extremely difficult to remove misfired cartridges from the hole, thereby endangering personnel.
These and other needs are addressed by the various embodiments of the present invention. The cartridge includes a number of advantageous features that individually and collectively provide a highly efficient and robust small charge breaking system.
In one embodiment, a small charge breaking system is provided that includes the following:
(a) a cartridge including:
(i) a housing;
(ii) a bulk energetic material contained in the housing, the bulk energetic material filling at least most of the volume enclosed by the housing; and
(iii) a high energy igniter operatively disposed relative to the bulk energetic material such that the igniter can initiate the low energy energetic material;
(b) a hole extending into a material to be broken by the bulk energetic material; and
(c) a stemming material positioned between the cartridge and an opening of the hole. The stemming material impedes the escape from the hole of gas released by the bulk energetic material after initiation of the bulk energetic material.
The bulk material can be any suitable energetic material, preferably releasing no more than about 1 megajoules/kg/millisecond and preferably deflagrating rather than detonating. The bulk material can be an explosive or nonexplosive material such as a propellant. Preferred materials include nitrocellulose, nitroglycerine, nitroguanadine, and mixtures thereof. When the bulk material is a propellant, the propellant will typically have a bum rate, in the cartridge, of no more than about 5 mm/second of propellant grain.
The high energy igniter can be of any suitable design. The igniter can be electronic, electric or nonelectric, can be adapted to incorporate electronic timing mechanisms to effectuate downhole delay, and can employ an initiator energetic material, such as a secondary energetic material, a pyrotechnic, or any combination of the foregoing. The igniter can include a microprocessor, timer, and memory to process commands regarding initiation delay time. Preferably, the high energy igniter, after initiation, releases a total amount of energy of at least about 250 joules and more preferably at least about 500 joules and even more preferably at least about 750 joules into the bulk energetic material. In one configuration, the igniter includes a pyrotechnic material, such as boron calcium chromate titanium (xe2x80x9cBCTKxe2x80x9d) potassium percarborate and is otherwise free of a primary or secondary energetic material. An example of such an igniter is disclosed in U.S. Pat. No. 5,710,390, which is incorporated herein by this reference. The high energy initiation preferably deflagrates and does not detonate. As will be appreciated, a detonator will have a poor ability to initiate a granular bulk energetic material such as a propellant.
In one configuration, a synergistic combination of the high energy igniter with a bulk energetic material that includes ammonium nitrate and a double based propellant provides unexpectedly effective results.
The stemming material can be any nonenergetic material that is stable under the conditions present in the hole during initiation and consumption of the bulk energetic material. The stemming material can be a consolidated material, such as a resin, a water-based gel, cementitious material, grout, concrete, metal or composite bar, and clay and/or an unconsolidated material, such as gravel, sand, and dirt. In one configuration, a stemming member is used to hold an unconsolidated stemming material in the hole. The unconsolidated stemming material has been found to absorb shock from the initiation of the cartridge and thereby protect the stemming member from damage.
To provide a low incidence of misfires and more efficient combustion of the bulk energetic material, the xe2x80x9chot endxe2x80x9d of the high energy igniter is preferably disposed in a portion of the bulk energetic material located in a middle section of the housing and spaced from opposing ends of the cartridge. The xe2x80x9chot endxe2x80x9d of the igniter refers to the end of the igniter from which a flame is emitted during initiation. The igniter is configured to xe2x80x9cshootxe2x80x9d a flame forward into the (bulk) propellant, resulting in a linear ignition rather than a point initiation. While not wishing to be bound by any theory, it is believed that, when the igniter is initiated, dual flame fronts propagate towards the opposing ends of the cartridge. By positioning the hot end of the cartridge in the middle of the cartridge, both flame fronts can simultaneously consume large amounts of the bulk energetic material, resulting in relatively high rates of pressure increase in the cartridge body and in the hole with concomitant high rates of rock breakage per cartridge compared to conventional cartridge designs. Preferably, the igniter preferably shoots the flame forward into the low energy secondary material. Preferably, the hot end of the igniter is positioned at least about 20% and more preferably from about 30% to about 50% of the cartridge length from the proximal (or distal) end of the housing.
In another embodiment, a cartridge is provided that includes:
(a) a housing having proximal, middle, and distal sections disposed along a length of the cartridge, the middle section being between the proximal and distal sections;
(b) a bulk energetic material contained in at least the middle section of the housing, the bulk energetic material filling at least most of the volume enclosed by the middle section of the housing; and
(c) an igniter operatively disposed relative to the bulk energetic material such that the igniter can initiate the energetic material. The igniter is positioned entirely in the middle section of the housing.
In one configuration, the housing has a body and opposing end caps. The end caps are discrete from the body. The end caps are removably attached to the body such that the end caps will separate from the body in the event of accidental ignition of the bulk energetic material.
To provide initial containment of the gases during combustion of the energetic material, the housing or cartridge body is preferably formed from a rigid or semi-rigid waterproof or water resistant material. Having too rigid or strong a cartridge housing, though desirably providing a high degree of initial confinement of the gas generated by combustion of the bulk energetic material, can cause higher risks of injury to personnel during cartridge transportation and result in compliance with more stringent regulations and therefore higher shipping costs. Having too flexible or weak a cartridge housing, though desirably providing a higher degree of safety and lower shipping costs due to compliance with less stringent regulations, provides poor initial gas confinement and less efficient rock breakage. Preferably, the cartridge housing material has a yield strength of at least about 500 psi and no more than about 3000 psi. Preferably, the cartridge can withstand external pressures of up to about 25 psi.
To provide effective sealing from downhole liquids such as water and consequent lower misfire rates and lower risks of other performance problems, engagement surfaces between one or both of the end caps and the body can contain multiple interlocking sealing surfaces. In one configuration, the interlocking sealing surfaces are serrated or undulating in profile.
To provide ease of assembly and maximize the amount of energetic material that can be placed in the housing, one or both of the end caps preferably fits over the corresponding mating surface of the body section.
To provide ease of insertion into the hole and permit removal of the cartridge from the hole in the event of a misfire, the exterior surface of one or both of the end caps is tapered away from the body section and the edges of the caps are rounded or arcuate. In other words, a distal end of the end cap has a smaller periphery than a proximal end of the end cap adjacent to the body section to define a frustoconical surface. The cartridge has a shorter effective length when inserted into the hole, has lower risks of being bound in the hole during insertion due to irregularities in hole width and/or bearing, can be readily autoloaded or mechanically loaded into the hole, and can be easily removed from the hole by a suitable downhole tool in the event of misfire (as the uphole end cap has sufficient clearance relative to the walls to permit a downhole tool to grip the cap).
To facilitate placement and alignment of multiple cartridges in the hole, a joiner sleeve can be employed. The joinder sleeve fits between and engages simultaneously adjacent downhole cartridges.
To protect the activator to the igniter from damage particularly during insertion of the cartridge into the hole, the housing can include a groove extending at least most of the length of the cartridge for receiving the activator or lead to the igniter, when the activator enters the cartridge in a downhole end of the cartridge. The groove can be have an arcuate or radiused portion that wraps around an end of the cartridge. The activator can be of any suitable configuration, such as a twisted wire, a (nonelectric) shock tube, and a circular section single or pair electrical cable.
To further protect the igniter from undesirable displacement in the cartridge body during insertion, the activator can enter an end of the housing at an entry hole that is located off-center relative to a longitudinal center axis of the cartridge. To seal against downhole liquids, a pliable waterproof or water resistant seal can be inserted into the entry hole to receive the activator.
In yet another embodiment, a small charge breaking method is provided that includes the steps of:
(a) providing a cartridge of any of the configurations described above;
(b) inserting the cartridge into a hole extending into a material to be broken by the bulk energetic material;
(c) inserting a stemming material into the hole such that the stemming material is positioned between the cartridge and an opening of the hole; and
(d) igniting the igniter to cause initiation of the bulk energetic material.
In yet another embodiment of the present invention, a method for manufacturing a cartridge for small charge breaking, comprising:
(a) at least substantially filling a cartridge housing with a bulk energetic material;
(b) locating an igniter in a middle portion of the cartridge housing;
(c) positioning a lead to the igniter along a groove extending a length of the cartridge housing; and
(d) engaging an end cap with an open end of the cartridge housing. The above steps can be performed at the excavation site.
The above summary is intended to be neither complete nor exhaustive. Other embodiments of the invention are obvious to one of ordinary skill in the art based on the teachings in the present application. Such other embodiments are intended to be included within the scope of the present invention.